This invention relates to an auto focusing apparatus of a scanning electron microscope.
In a scanning electron microscope, as shown in FIG. 1, an electron beam generated from an electron gun (not shown) is scanned by deflection coils 2X and 2Y and focused into a narrow beam, which in turn is irradiated onto a specimen 4. Secondary electron 14 generated from the specimen 4 is detected by a detector 5. By using an electron signal amplified by an amplifier 6 as a brilliance modulation signal for a monitor (not shown) and by synchronizing the signal with scanning by the deflection coils 2X and 2Y, the brilliance modulation is complete and a scanning electron microscopic image of the specimen 4 is obtained on the monitor screen. Stage 15 can receive coordinates data of a measuring point from a wafer information file stored in an external storage device, in which the coordinates of points to be measure are registered, and move the wafer to a required measuring point. Data is registered in the wafer information file for each production process of wafers such as a gate creation or contact hole creation process or for each type of wafer, such as 1M-bit memory or 4M-bit memory wafers, and the file is loaded by an operator from the external storage device before measuring each wafer.
In the meantime, because the absolute value of time differential (or position differential) of the electron signal detected by the detector 5 becomes larger as the electron beam 1 is focused on the specimen 4 more accurately, it can be used as an index for evaluating the focused condition. An area within the dotted line in FIG. 1 shows an example of a conventional focused position detector 10 which employs this index. For performing focusing, as the exciting current of the objective lens (electro-magnetic lens) 3 is changed sequentially and gradually by a focus controller 7, each exciting current makes the electron beam 1 scan on the specimen 4. Then, the intensity of the secondary electron signal obtained at each excitation is integrated and the absolute value of the signal for a certain scanning period is differentiated, both by a signal intensity integrator 8. As a result of the above calculation, a value corresponding to each focused condition is obtained as an output from the signal intensity integrator, which is now called a focus evaluation value. In an understanding that the beam is exactly focused when the focus evaluation value obtained under each excitation of the objective lens reaches its peak, a peak detector 9 detects the peak of the focus evaluation value, and an exciting current that makes the focus evaluation value become the peak is sent to the objective lens 3 from a focus controller 7, thereby performing focusing.
Next, a brief explanation is given, using FIG. 2, about a conventional auto focusing method that employs a picture processing technique. In FIG. 2, the same functional parts as in FIG. 1 are given with the same numbers and a detailed description on them is omitted. Using the secondary electron signal from the amplifier 6 as a brilliance modulation signal of the monitor 11, which is scanned in synchronization with the deflection coils 2X and 2Y, a scanning electron microscopic picture of the specimen 4 is displayed on the monitor 11. Because the contrast of the picture becomes more intense as the electron beam 1 is focused on the specimen 4 more exactly, the picture signal serves as an index for evaluating the focus when the signals of adjacent picture elements are integrated or differentiated and the sum of their absolute values is calculated. Here, the sum is called a focus evaluation value. An area within the dotted line in FIG. 2 shows an example of a focused position detector 10 which employs this focus evaluation value. The function of the focused position detector 10 is the same as in FIG. 1 except that a picture processor 8 receives the picture signal from the monitor 11 and calculates the focus evaluation value while the signal intensity integrator 8 calculates the value in FIG. 1. Similarly as in FIG. 1, the peak detector 9 detects the peak of the focus evaluation value and an exciting current that makes the focus evaluation value become the peak is sent to the objective lens 3 from the focus controller 7, thereby performing focusing.
While determining the current of the objective lens in auto focusing in the aforementioned prior art, because the focus evaluation value depends on the integrated value of the secondary electron signal intensity due to the principle of its operation, the focus is directed to a point where the intensity of the signals detected in the scanning area changes the most. Because the signal intensity exhibits higher contrast particularly at an edge along the height of the specimen, an unstable peak of the focus evaluation value may appear when the current of the objective lens is changed gradually or a focus is determined at a height of the specimen where particularly intense contrast is resulted if differences in height are included in the scanning area of the specimen. As a result, the point determined by the auto focusing is sometimes different from the point the operator really wants to observe.
In a mass production factory for semiconductor wafers, a scanning electron microscope designed for semiconductor wafers is employed to control pattern dimensions of the wafers after specific production processes. In this application, a point where the dimensions need to be controlled is predetermined and the dimensions of the specific point is always measured on every mass-produced wafer for quality control. Although patterns to be controlled include the line width and the hole pattern diameter, later production processes involve more complicated differences in height on a wafer. Further, in measuring the inside diameter of a bottom of the hole pattern or measuring the dimension of a bottom of a line pattern, the signal from the edge is more intense while that the signal from the bottom is less intense. Thus, an auto focusing mechanism frequently directs the focus to a height which the operator does not intend to measure. When this mislocation of the focus occurs, it is necessary to correct the focus manually for all wafers after execution of auto focusing operation because the measurement of the pattern dimensions of the wafers are always done on the same point. Manually correcting the focus is very inconvenient. If all of the control processes are to be carried out automatically, and if the mislocation of focus occurs then the measurement is done on an incorrectly focused point and accordingly the obtained data is less reliable, which is a serious quality control problem.